Glass-liquid transition of carbon dioxide and its effect on water segregation

The interactions between CO2 and D2O molecules have been investigated by using time-of-flight secondary ion mass spectrometry in the temperature rage 13-120 K. The monolayer of CO2 tends to wet or intermix with the D2O film below 40 K and dewets the surface above 60 K. The water nanoclusters deposited on the CO2 multilayers also start to segregate at 50-60 K and are finally incorporated in the bulk at 85-90 K, where the morphology of the film changes abruptly together with the desorption rate of the CO2 molecules. The break at 85 K should be caused by the occurrence of the fluidized film whereas the glass-transition temperature of CO2, as determined from the onset of translational molecular diffusion, is assigned to 50 K. This behavior may be related to the ultraviscous nature of the supercooled liquid, arising from the decoupling between the translational molecular diffusion and viscosity. The He+ irradiation of the mixed CO2-D2O ice and the D-2(+) irradiation of the CO2 ice at 13 K do not yield any surface residues assignable to H2CO3 and its precursors above 100 K. This result may be related to the segregation between the CO2 and D2O molecules.